Thermionic amplifier



'Jan- 1948- I P. K. CHATTERJEA EIAL 4, 8

THERMIONIC AMPLIFIER Filed Jan. 12, 1944 y u M Atto ney Patented Jan. 27, 1948 UNITED PATENT OFFICE rsculiyr L n on England, assignors, by mesne,

assignments, to.{lnternationalStandardjElectric C,orporation,..1\,lework, N. v"5L, acorporation of Delaware .LApplicationsIanuaryTIZ, 1944,,Serial No. 517,914

imGIeaL-Britain January 13, 19,43

' 16 "C a m .1

The present invention relates to thermionic valve circuits and inmarticular to audioirequency amplifying: circuits such as are. usedin radio rre- Lceivers.

.The invention makes ;use i of, the properties 10f devices; known; as thermistors.

[I'herm-istors are thermfiwnsitive; resistance elements having a temperature coefficient of resistancewhich may he eit enuosi iv o ine at v aud lwhichris more ver.menyzt mesathe co -respond n coeflic e t f or; a pur .--meta1 suchas c nve -;'I'his pr pe ty-r d rs t r istor p rt ularlvzsuitable for a vari y of sp a applicat, .ns.:inee1e tri circuit Variousdifiere materia s ar -all lablerforzthe resistance element ef 1a thermistor, zthesezvaricus materials havin dlifierent propertiesiin other-re- :spects; as one example, ,azresistance-material havin a. highsnegativeytempemturecoeflicient :of resistance comprises firtlfliXtliI'fl of :manganese oxideand niokelcoxide, with orwithout-:theaddi- :tiomof certain;othenmetallic-oxides; the mixture being suitably heat treated.

-Thermistprs:haverbeentemployed"in:two diifer- :ent forms: :(ia) known a directly heated thermistor and comprising a resistance element of the-thermally sensitiverresistancematerial p1 0- videdswith; suitable :leadi-out conductors orterminals, and (7)) known as .-an indirectly heated thermistor comprising theielement (a) provided in :aolditionv 1 with a heating 'coil electrically-insulated :from the element. A directly heated thermistor is primarily-intended to be controlled by the current-which-flows through-it-and which varies *the tempe-rature and also the --resistance accordingly. Such a thermistor will also be affected by the temperature ofitssurroundingsand may therefore be used for 'thermostatic ;-control and like'purposes wither without direct'heatingby the-currentflowing-through it. Anindi rectly heated thermistor js -chiefly designedto be heated by a controlling current which flows through "the heating coil and which will usually, but not necessarily, be difierentirom: the current which flows -'through the "resistance ,elernentgbut this-type-of-thermistor may 5 150106, 11biecte to either or; both ,ofthe types ,of control applicable to a direc y: eatedtthermistorore detailed information =p.11 t h rn ceerties not rep du d. a a l.

' 2 pf thermistors will befound-in an article-by G.-L. Pearson in the Bell Laboratories Record, Dec. 1940, page 106.

The present invention is concerned with the improvement of the performance of audio frequency reproducing systems, particularly those used in radio receivers of "the cheaperikinds. In such receivers articularly in ompact types, t is necessary to economise in space, 00st, and power consumption, andtherefore the loudspeaker and the power available to operate itare sinall. The low frequency end of the speech frequen y band is accordingly attenuated acoustically, and may In these circumstances it-isuseless to raise thelevel of the .loW frequency end of the range electrically, as very little improvement acoustically. could beproduced this way.

It is well known that if several harmonicsrof a fundamental frequency are simultaneously app ed to the human. ear acou ca y at; s ffi e t y 'high level, thefundamentalis generated therein on account of asymmetric distortion inthe sound transmission. Use ismade of thispropertyto improve the apparent qualityof cheap radio receiversor other apparatus employing an audio frequency reproducing system, by generating harmonics of thelow frequencies inlthe power output stage (or anywhere else in the voice-frequencypart of the circuit) and these are suppliedto the loudspeaker which transmits'them at a reasonable level. The earthen-manufactures the fundamentals from which the-harmonics-were ;derived, .,and,the absence ofthe low frequencies, which would. otherwise cause thereproduction to sound DOQLiS not noticed.

Thearrangementspreviously used for generating, harmonics-have involved theuse of valves andfilters. Thelobjectof the-present invention is,. .t1; simplify the production-of the harmonics by, theuse-of a directly-heated thermistor.

According to the invention, therefore, there is prov,i,ded an electrical reproducing ,circuit ,wherein the loudspeaker or other reproducing device is operated by a circuitcomprising a theranion c va v ampl fier.cha cter sed n t i t t a directly heatedthermistor L is connected to the amplifier fi l-15 w; that. cu e ts of si .ieequencies p ,d athrqueh the 'ithermiston-the characteristic of which and the operating point thereon being so chosen that the thermistor genplayed to explain the action of Fig. l; and

Fig. 3 shows a schematic circuit diagram of another embodiment of the invention. I

Fig. 4 shows a thermistor connected with a low pass filter.

Fig. 1 shows the last voice-frequency power stage of a radio receiver, and will be seen 'to be a particular case of Fig. 1 of British application No. 12,884/42. A thermionic valve V has the loudspeaker LS connected in series with the anode,

and the resistance element R of a directly heated thermistor T, which may be shunted by a con-- denser C is connected in series with the cathode. The purpose of this condenser is explained below, but its use is not essential, and it may be omitted if desired. X2 is the grid leak resistance, and K is a blocking condenser of suitable capacity through which the incoming signals are applied to the control grid of the valve V .from other parts of the radio receiver (not shown).

Fig. 2 shows two typical characteristic curves for a directly heated thermistor having a, negative temperature coefiicient of resistance. The curve i shows the relation between the resistance value of the element R and the current flowing through it, and the curve 2) shows the relation between the resistance and the voltage across it. The 2; curve indicates that there is a critical maximum M above which the voltage. cannot rise, and that there are two possible values of resistance corresponding to any voltage less than this maximum value. The part LM of the curve 12 corresponds to an unstable condition; and the current tends to increase spontaneously if a voltage equal to or greater than .the critical value is applied.

Thethermistor should be so chosen that when no signals are being received, the current and resistance in the circuit of Fig. 1 correspond to the point P on the i curve which lies on an ordinate PN which cuts the stable part of the 0 curve at a point Q. The condenser Ciif used) is chosen so that it effectively short-circuits the thermistor except for the frequencies near the low end of the signal frequency range. Thus when a low signal frequency is received, the anode current fluctuates accordingly and the fluctuations are permitted by the condenser to pass through the thermistor. The point P in general describes an oval curve as indicated owing to the delay in response of the thermistor, this oval curve tending to coincide practically with the curve 11 for the lowest frequencies. The point Q describes a similar oval curve which represents voltage variations which are fed back to the control grid of the valve to be mixed with the incoming signals. Owing tot-he non-linear response of the thermistor, harmonics will thus be generated and mixed with the signals, and will be reproduced by the loudspeaker.

Although in Fig. l the thermistor is connected in series with the cathode of the valve for the purpose of generating'harmonics of the signal frequencies present, similar results can be ob- 4 tained by connecting the thermistor in other parts of the amplifier which feeds the loudspeaker.

In Fig. 3 is shown part of the final amplifier of a radio receiver comprising two thermionic valves V1 and V2 the cathodes of which are biassed in. the usual way by means of appropriate series re-- sistances X each shunted by a condenser Q. The loudspeaker is connected in series with the anode of the last valve to the high tension supply E, and the "anode of the valve V1 is supplied from E' through the resistance element R of a directly heated thermistor T and through a resistance R1.

The anode of V1 is connected to the control grid of V2 through a blocking condenser K and the usual grid leak resistance R2 is provided. The thermistor may be shunted by the condenser C whose purpose is as described above.

The thermistor should be so chosen that with the normal anode current of the valve Vi the operating pointis at Q in Fig. 2 on the stable portion of the characteristic curve. The harmonics generated in the thermistor will produce corresponding variations of the anode voltage of V1 which are supplied directly to the control grid of V2 and thence to the loudspeaker.

According to another variation the thermistor T together with its shunting condenser C (if used) may be removed from the position shown in Fig. 3, and connected instead to replace the whole or part of the resistance R2. In this case, since there is no direct current flowing through the thermistor the operating point will be at S in Fig. 2. Harmonics will be generated partly on account of the curvature of the characteristics and partly on account of the fact that both halfwaves will produce a reduction of the thermistor resistance, which is equivalent to the introduction of even harmonics.

It will be understood that the condenser C does not sharply divide the range into portions so that only certain low frequencies generate harmonies. Actually the effect produced is that the harmonics are produced at greater amplitude as the frequency becomes lower, which is the condition desired, and at some not too high predetermined frequency (say several hundred cycles per second) the generation of harmonics has become inappreciable.

If it is desired to limit the frequency range over which harmonics are generated very sharply, the condenser C may be dispensed with and a suitable low pass wave filter provided as shown in Figure 4 through which the thermistor is connected to the circuit of Fig, 1 or Fig. 3. This filter can be designed so that frequencies above a certain predetermined frequency substantially do not reach the thermistor, so that harmonics cannot be generated by them.

It will be noted also that the fundemental low frequencies tend to be reduced in intensity by the arrangements of Fig, 1 or 3 but this is not of much importance since the loudspeaker could not reproduce them any way.

It will be evident that the thermistor can be arranged as described with reference to Fig. 1 or 3 in connection with any part of the audio frequency amplifier of the radio set. In other words, it is not essential, in the case of Fig. 1, for example, that the thermistor should be connected in series with the cathode of the last valve. It would operate equally well if connected in series with any other audio frequency valve, and similarly for Fig. 3. Furthermore, a thermistor with a positive temperature coeiiicient of resistance may be used, since it will have a curved characteristic adapted to generate harmonics, though it will not have a negative slope in any part. Accordingly the operating point may be chosen anywhere on the characteristitc. Also, the final utilisation means connected to the output of the amplifier need not be a loudspeaker; it could be a pair of head-phones or other electromechanical device.

What is claimed is:

1. An electrical reproducing circuit of the type wherein an electro-acoustical transducer is operated by current derived from a thermionic valve amplifier, including a directly heated thermistor connected in series with the cathode of said thermionic valve so that the cathode current of said valve flows therethrough, the thermistor characteristics and the operating point thereon being so chosen that said thermistor generates harmonics of signal frequencies at the lower end of the voice frequency range for application to said reproducing device.

2. Electra-acoustical transducing circuit including an electronic amplifier, a sound reproducer in the output circuit thereof, and a thermistor having a negative temperature coefiicient and adjusted so that in the absence of signals the current which flows therethrough corresponds to a point on the stable part of the voltage-current characteristic curve of said thermistor, said thermistor being connected in series with the cathode of said thermionic valve so that the cathode current of said valve flows therethrough, whereby said thermistor generates harmonics of signal frequencies at the lower end of the voice frequency range for application to said reproducing device.

3. An electrical reproducing circuit of the type wherein an electro-mechanical sound reproducing device is operated by the output of a thermionic valve amplifier, including a directly heated thermistor connected to said amplifier in such manner that currents of signal frequencies pass through said thermistor, the characteristic of said thermistor and the operating point thereon being so chosen that the thermistor generates harmonics of signal frequencies at the lower end of the voice frequency range for application to said reproducing device, also including a condenser shunting said thermistor, the capacity of said condenser being chosen substantially to short-circuit said thermistor at and above a predetermined frequency.

4. An electrical reproducing circuit adapted to be connected with an electro-mechanical sound reproducing device comprising a thermionic valve having a cathode, anode and a control grid, a directly heated thermistor connected to said valve in such manner that currents of signal frequencies pass through said thermistor, the the characteristic of said thermistor and the operating point thereon being so chosen that the thermistor generates harmonics of signal frequencies at the lower end of the voice frequency range for application to said reproducing device, the thermistor being connected in series between the control grid and the cathode of the valve.

5. An electrical reproducing circuit of the type adapted to be connected with an electro-mechanical sound reproducing device comprising a thermionic valve having a cathode, anode and control grid, a directly heated thermistor connected to said valve in such manner that currents of signal frequencies pass through said thermistor, the characteristic of said thermistor and the operating point thereon being so chosen that the thermistor generates harmonics of signal frequencies at the lower end of the voice frequency range for application to said reproducing device including means connected to said thermistor for preventing appreciable generation of harmonics for frequencies above a predetermined frequency applied to said thermistor.

6. An electrical reproducing circuit of the type wherein an electro-mechanical sound reproducing device is operated by the output of a thermionic valve amplifier, including a directly heated thermistor connected to said amplifier in such manner that currents of signal frequencies pass through said thermistor, the characteristic of said thermistor and the operating point thereon being so chosen that the thermistor generates harmonics of signal frequencies at the lower end of the voice frequency range for application to said reproducing device, and a low pass filter so connected with the thermistor as to pass through said thermistor substantially only currents below said predetermined frequency.

PRAFULLA KUMAR CHATTERJEA. CHARLES THOMAS SCUILY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,313,096 Shepard Jr. Mar. 9, 1943 2,017,192 Wolff Oct. 15, 1935 

